This invention relates to the field of electrochemical analytical instruments and, more particularly, to an improved oxygen electrode which provides consistent, reproducible and reliable signals and is resistant to poisoning by certain ions or by the components of biological systems.
When used as a cathode in a polarographic, galvonometric or amperometric analytical system, an oxygen electrode is effective for the reduction of oxygen in its environment and provides a current or voltage output which is a function of the oxygen concentration in that environment. Oxygen electrodes are constructed of conductors such as platinum or gold which catalyze the cathodic reduction reaction. Under a negative voltage of 0.3-0.8 volts with respect to a saturated calomel electrode, oxygen may be reduced at a bare platinum electrode, for example, in accordance with the following reaction: ##EQU1## In certain environments, most particularly the biological systems whose oxygen content such electrodes are often used to measure, the electrode "ages" rapidly and loses its ability to catalyze the second step of the four electron reduction set forth above. The electrode reaction thus becomes increasingly limited to the two electron reduction illustrated in the first step of the equation and the current output declines.
While the shift from four electron to two electron transfer would not present a serious obstacle to reproducible analyses if the shift were consistent, quantitative and predictable, the available oxygen electrodes have exhibited a proclivity for random shifts in their current outputs both upward and downward. As a consequence, their practical utility for measuring oxygen concentrations (without recourse to frequent recalibrations) has been rather limited.
In addition to the shifts in current output resulting from electrode aging, the progress of the oxygen reduction reaction taking place at the electrode is adversely affected by various poisons. Certain ions are poisons and the macromolecules contained in biological systems have a further unfavorable impact on the predictability and reproducibility of operation of oxygen electrodes. Proteins, in particular, are known poisons for oxygen electrodes and those proteins containing sulfur bearing amino acids are especially deleterious.
In an effort to avoid such aging and poisoning problems, the so-called Clark electrode has been developed in which a glass insulated platinum conductor is immersed in a standard electrolyte that is in turn contained within a membrane that separates the standard electrolyte and conductor from the environment in which it is used. The membrane has such diffusional properties as to permit measurement of the oxygen content of the electrode's environment. However, the Clark electrode is somewhat bulky, has a relatively sluggish response, and is not particularly well adapted for the long-term measurement of intravascular oxygen content or the oxygen content of tissue.
Efforts have also been made to protect oxygen electrodes from poisoning by dip coating the sensor in a polymeric material and allowing it to dry. However, it is very difficult to obtain a precise or uniform coating by a dipping technique, and the polymers that have been available for use therein have been susceptible to peeling away from the electrode surface. Moreover, the membranes produced by dip coating, and those utilized in Clark electrodes, have been relatively thick, thereby extending the response time by inhibiting the transport of oxygen to the sensing surface. As described in Hahn et at., "Plasma-Formed Polymers for Biomedical Applications Part II. Biocompatibility and Applications," National Bureau of Standards Special Publication 415 (May 1975), attempts have also been made to provide a membrane over a platinum-iridium oxygen electrode through deposition of a polymer coating by glow discharge polymerization. However, the coatings produced in that work were relatively thick, gave unsatisfactory electrochemical responses and prematurely peeled away from the metal electrode surface.